1. Technical Field
The present invention relates to an optical switching device (light valve) for use in optical communication, optical processing, optical memory devices, optical printers, image display devices, etc., and more particularly, to an optical switching device suitable for use in an optical image display device and to an optical image display device.
2. Background Art
Conventionally, an optical switching device using a liquid crystal is known in the art. FIG. 43 schematically illustrates an optical switching device 900 according to a conventional technique. As shown in FIG. 43, the optical switching device 900 includes polarizers 901 and 908, glass plates 902 and 903, transparent electrodes 904 and 905, and liquid crystals 906 and 907. In this optical switching device, optical switching is performed by applying a voltage between the transparent electrodes so as to modify the orientations of liquid crystal molecules thereby rotating the polarization plane. A conventional image display device is produced by disposing such optical switching devices (liquid crystal cells) in a two-dimensional fashion into the form of a liquid crystal panel. In this image display device, representation of gray scales is accomplished by controlling the alignment of liquid crystal molecules by adjusting the applied voltage.
However, liquid crystals are slow in response. The response time of liquid crystals is several msec at best. This makes it difficult to employ optical switching devices using a liquid crystal in applications which need a short response time, such as optical communication, optical processing, an optical memory such as a hologram memory, an optical printer, etc. Another problem of optical switching devices using a liquid crystal is a reduction in the light utilization efficiency caused by a polarizer.
In the art of the image display device, increasingly high image quality is required. To meet such a requirement, it is needed to provide an optical switching device capable of representing gray scales more precisely than optical switching devices using a liquid crystal.
Thus, it is an object of the present invention to provide a low-loss optical switching device capable of responding at a high speed. It is another object of the present invention to provide an optical switching device capable of forming a high-quality image with a uniform contrast.
According to the present invention, to achieve the above objects, there is provided an optical switching device which extracts evanescent light from a light guide capable of transmitting light by means of total reflection, wherein extraction of light is performed when a light-transmissive extraction plane of a switching part is brought into contact with the light guide. Light can be turned on and off at a high speed by moving the switching part by a small distance nearly equal to one wavelength or less. The switching part is formed to be of a reflective type. The light guide, the optical switching part, and driving means for driving the switching part are disposed in this order in a direction in which light is illuminated, thereby forming the optical switching device in a multilayer structure. Thus, an optical switching device can be realized which can output a large amount of light with a low loss and which can respond at a high speed. That is, the switching part according to the present invention includes: the light guide with a total reflection plane capable of transmitting light by means of total reflection; the switching part including a light-transmissive extraction plane capable for extracting evanescent light leaking through the total reflection plane; and driving means for moving the switching part to a first position at which the extraction plane is within an extraction range which allows an evanescent light to be extracted and a second position at which the extraction plane is out of said extraction range, wherein the light guide, the switching part and the driving means are disposed in this order in a direction in which light is output.
In the optical switching device according to the present invention, the light guide, the switching part, and the driving means for driving the switching part may be disposed one on another in this order into a multilayer structure in which the functions of the light guide, the switching part, and the driving means are implemented in the respective layers substantially independently of one another. This makes it possible to easily optimize the respective parts. In the optical switching device according to the present invention, light is reflected by the switching part toward the light guide, and no light passes through the driving means. Therefore, it is possible to optimize the driving means without having to consider optical conditions.
This allows the switching part to be supported on the driving means, and thus it becomes unnecessary for the light guide to have a structure on the side facing the optical switching part. Thus, the light guide can be constructed in the form of a simple flat panel. Because the light guide is not required to include a part for supporting the switching part, the entire area of the light guide on the side where the total reflection plane is formed can be used as a plane through which light is extracted, that is, as a plane the switching part is brought into contact with. Thus it is possible to realize an optical switching device having a large aperture area through which light is output, that is, having a large aperture ratio, which allows a large amount of light to be output. Furthermore, by employing the panel structure, it becomes possible to use a plane opposite to the total reflection plane as an output plane through which the extracted light is output to the outside. The driving means may be formed on an integrated circuit substrate for controlling the driving means. The optical switching device may be integrated with an integrated circuit chip for controlling an image.
A pixel may be formed by one or a combination of a plurality of optical switching devices according to the present invention. If a plurality of optical switching devices are disposed in a two-dimensional fashion, and if the light guide is adapted to be capable of transmitting white light or three primary color light, an image display device can be obtained. This image display device can display a high-resolution image at a high speed. Furthermore, the image display device can be constructed into the form of a multilayer structure, which allows the image display device to be produced at low cost. The image display device may be integrated with an integrated circuit chip.
To realize the switching part of the reflective type, a light outputting member for outputting extracted light may be formed using a microprism or a light-dispersive material such that light extracted through the extraction plane is reflected by the light outputting member. The direction of the output light may be controlled so that light is output in a direction substantially perpendicular to the total reflection plane.
The driving means may include a supporting member for elastically supporting the switching part and electrostatic driving means for driving the switching part by means of electrostatic force acting between a pair of electrodes whereby the switching part may be moved to the first and second positions. Preferably, the supporting member urges the switching part at the first position toward the light guide when no electrostatic force is exerted by the electrostatic driving means. The electrostatic driving means can be easily controlled by electric power. However, the driving force provided by the electrostatic driving means varies with a variation in voltage or current. In contrast, a driving force provided by an elastic element is stable because it is generated mechanically. In view of the above, the supporting member formed of an elastic material is employed to stably generate a driving force thereby urging the extraction plane of the switching part into close contact with the total reflection plane of the light guide so as to turn on the optical switching device, whereas a driving force supplied by the electrostatic driving means which can be easily controlled by electric power is employed to move the extraction plane away from the switching part so as to turn off the optical switching device. This makes it possible to provide an optical switching device having high controllability and capable of stably outputting a desired amount of light.
Preferably, the supporting member is adapted to have a residual deformation when the switching part is at the first position so that the extraction plane is urged, in the on-state, by the elastic force of the supporting member against the total reflection plane of the light guide thereby ensuring that the extraction plane is in intimate contact with the total reflection plane. This makes it possible to provide an optical switching device having high brightness in the on-state and high on-off contrast. Furthermore, the residual deformation of the supporting member makes it possible to accommodate a variation in the distance between the light guide and the switching part or a variation between the switching part and the driving means due to vibrations, a temperature variation, or aging effects.
The switching part is preferably supported by the supporting member via a spacer. The spacer may serve to reduce the distance between electrodes and provide a space in which the supporting member is allowed to be elastically deformed. Such a spacer may be formed in a T shape or inverted trapezoidal shape in cross section with the driving means located below. The reduction in the distance between electrodes allows a reduction in the driving voltage and an increase in the operation speed.
By employing a spacer having such a shape, it becomes possible to employ, as the supporting member, a spring member in the form of a plate one end of which is supported by a post located near the boundary of the switching part and the other end of which is connected to the switching part. This makes it possible to realize a spring member having a large effective length whereby the force for urging the switching part may be properly adjusted. Thus, it is possible to adjust the elastic force such that on/off operations can be performed in a highly reliable fashion even if the driving force supplied by the electrostatic force is small. Furthermore, the employment of the T-shaped or inverted trapezoidal-shaped spacer allows the spring member to have a sufficiently large effective length without causing a reduction in the area of the switching part. Therefore, it is possible to provide an optical switching device having a large aperture area through which light is output. A plurality of such optical switching devices may be employed to form an image display device in such a manner that optical switching devices are located in a seamless fashion substantially without being spaced from each other.
The spring member may be formed in an arbitrary shape such as a coil shape. However, if the spring member is formed such that its one end is supported by a post located near the boundary of the switching part (device) and the other end is connected to the optical switching part, then it is possible to position the optical switching part by the spring member. In this case, if the spring member is formed in the shape of a plate having a slit or opening near the boundary, interference among adjacent optical switching devices can be prevented. Furthermore, it also becomes possible to adjust the elastic modulus of the spring member to a value optimum for driving the switching part.
More specifically, such a plate-shaped spring may be formed to have a small width and radially extend from the optical switching part wherein one end of the spring member is supported by the post disposed near the boundary. In addition to such a spring member, the electrode may be formed in such a manner as to radially expand from the switching part thereby expanding the area of the electrode. This makes it possible to obtain a large driving force using a small voltage. That is, it is possible to reduce the driving voltage. The spring member may also be in the form of a plate including a part extending in a spiral fashion along the boundary. This allows the effective length of the spring to be increased without increasing the area thereof Thus, it is possible to reduce the voltage required to drive the optical switching part thereby reducing power consumption. The spring member may also include parts extending in a double spiral fashion. The spring member may also be formed in such a manner that the bending part of the spring (the central part between two supporting points) has a thickness smaller than the other parts so that the spring member has a reduced elastic modulus which provides effects equivalent to those obtained by increasing the effective length of the spring. When an image display device is formed using a plurality of optical switching devices, a plurality of plate-shaped springs serving as supporting members may be disposed at regular intervals near boundaries between adjacent optical switching devices such that supporting members may be shared by adjacent optical switching devices.
The post may be a protrusion extending along a long length at the boundary between adjacent optical switching devices. Alternatively, a plurality of posts may be disposed at proper intervals along the boundary. This allows a reduction in space occupied by the posts, and the remaining space can be used for the electrode or another element. The posts may be disposed at random locations or at locations determined according to a particular rule so as to provide a high-stability optical switching device and an image display device having a symmetric structure which can be easily assembled.
The spring member may be formed using an electrically conductive thin-film such as a boron-doped silicon thin-film such that the thin-film also serves as an electrode of the electrostatic driving means.
An auxiliary post may be disposed between the light guide and the spring member. In this case, if the spring member is formed in the shape of a plate having neither a slit nor an opening, as opposed to the above-described example, then it is possible to enclose the side where the switching part is disposed in a substantially hermetic fashion such that the side where the switching part is disposed has a lower pressure than the side where the driving means is disposed, whereby the spring member is pressed by the ambient pressure against the auxiliary post thereby ensuring that the gap between the switching part and the light guide is maintained uniform. Furthermore, the pressure difference allows the switching part to be in intimate contact with the light guide when the switching part is in the on-state. This makes it possible to provide a high-contrast optical switching device capable of operating in a stable fashion.
If the inside of the optical switching device with the switching part includes a hermetically enclosed space, and if the driving means is disposed in the hermetically enclosed space, then it is possible to reduce the pressure in the enclosed space or replace air in the enclosed space with gas such as an inert gas with a low pressure. This allows a reduction in the gas flow resistance during a switching process. As a result, friction with gas due to a damper effect or the like is greatly reduced. Therefore, it is possible to reduce the driving voltage and increase the driving speed. Furthermore, it is possible to perform on/off switching operations at a high speed. Thus, it is possible to provide an optical switching device capable of responding at a high speed.
Driving the Switching Part
In the optical switching device in which the switching part is driven by a combination of elastic and electrostatic forces, it is important that the switching part can be driven by a driving voltage which is as small as possible.
If the distance along which the switching part moves in each on-to-off or off-to-on switching operation is denoted by d, and the driving voltage is denoted by Vd, then the elastic force Fg and the electrostatic force Fs exerted on the switching part during the switching operation can be represented as follows:
Fg=Kxc3x97xxe2x80x83xe2x80x83(1)
Fs=Cxc3x97Vd2/(dxe2x88x92x)2xe2x80x83xe2x80x83(2)
where x is the moving distance of the switching part, K is the elastic modulus of the supporting member, and C is a constant which is proportional to the area of the electrode and which depends on the dielectric constant. When the switching part is at rest after the on/off switching motion, the elastic force Fg and the electrostatic force Fs are balanced. Therefore, to reduce the driving voltage Vd, it is desirable to reduce the elastic force Fg. A reduction in the moving distance d is also desirable. However, if the elastic modulus K is reduced, the moving speed of the switching part decreases. As a result, a reduction in the response speed occurs. On the other hand, if the moving distance x is reduced, it becomes difficult to obtain a sufficiently high on-off contrast. For the above reason, it is difficult to reduce the driving voltage Vd. If the elastic force Fg and the electrostatic force Fs are balanced at the on- or off-position, there is possibility that the attitude of the switching part becomes instable and degradation of the light modulation performance occurs. In switching devices using an evanescent wave, even a slight gap between the total reflection plane and the extraction plane can cause a reduction in the amount of extracted light.
In view of the above, the present invention provides a technique of driving the switching part in a highly reliable fashion at a high speed so as to obtain high optical contrast, using a reduced driving voltage Vd under the same conditions regarding the moving distance d and the elastic modulus K.
To this end, in the present invention, a fixed bias voltage with the same polarity as that of the driving voltage is applied between the electrodes by which the switching part is driven, thereby reducing the voltage required to drive the switching part. Furthermore, in order to hold the switching part at the on-position in a stable fashion under the application of the bias voltage, the holding force at the on-position is set to a value greater than the force created by the bias voltage. To this end, there is provided driving control means for applying to the electrostatic driving means a driving voltage for driving the switching part and a fixed bias voltage which is equal in polarity to the driving voltage and which provides a holding force capable of stably holding the switching part at least at the first position by means of electrostatic force or elastic force. The invention also provides a method of controlling a spatial optical modulation device, comprising a control step for applying to the electrostatic driving means a driving voltage for driving the switching part and a fixed bias voltage which is equal in polarity to the driving voltage and which provides a holding force capable of stably holding the switching part at least at the first position by means of electrostatic force or elastic force.
The application of the fixed bias voltage allows a reduction in the driving voltage which is applied to drive the switching part, and thus allows a reduction in the power supply voltage applied to the driving control means. Therefore, the control circuit serving as the driving control means is allowed to have a lower breakdown voltage, and it is possible to construct the control circuit in a simpler configuration. Furthermore, the power consumption can be reduced. By providing a holding force large enough to hold the switching part at the first position, it is assured that the switching part is maintained in a stable state with respect to the orientation when the bias voltage is applied. This allows the bias voltage to be continuously applied even when the switching part is at the first position. This means that the control of the bias voltage is not needed or can be simplified.
Preferably, there is provided a stopper for assuring a minimum gap between the electrodes at one of the first and second positions at which the holding force is provided by the driving voltage thereby preventing the electrostatic force generated by the bias voltage from becoming infinitely large when the switching part is at the first or second position and thus assuring that the electrostatic force is within a predetermined range. Preferably, the bias voltage is selected such that the electrostatic force generated by the bias voltage when the switching part is at the stopper position is smaller than the elastic force of the supporting member. This allows the switching part to be moved only by turning on and off the driving voltage. That is, it is allowed to maintain the bias voltage at a fixed value without controlling the bias voltage.
Alternatively, the bias voltage may be periodically applied such that the electrostatic force generated by the bias voltage becomes smaller than the elastic force of the supporting member when the switching part is at the first or second position. More specifically, the bias voltage may be changed in synchronization with an operating clock signal so that the switching part is allowed to be moved, at particular times corresponding to the timing of the operating clock signal, from the first or second position by the elastic force of the supporting member. Therefore, only by varying the bias voltage at predetermined intervals without having to control the bias voltage in synchronization with the driving voltage, it is possible to move the switching part in response to the driving voltage. This makes it easy to control the bias voltage. Furthermore, this technique also allows the bias voltage to have a value greater than the elastic force of the supporting member. Thus, it is possible to further reduce the driving voltage.
Furthermore, because the employment of the stopper for assuring the minimum gap between the electrodes assures that the electrostatic force created by the bias voltage falls within a predetermined range, it is possible to move the switching part in response to the driving voltage if the bias voltage is periodically changed to a value so that the electrostatic force created by the bias voltage at the stopper position becomes smaller than the elastic force of the supporting member. This makes it possible to limit the variation of the bias voltage within a small range. As a result, it is possible to form the circuit for controlling the bias voltage in a simple fashion, and a reduction in power consumption can be achieved.
The switching part may be moved by the supporting member from the second position to the first position and may be held at the first position by the elastic force provided by the supporting member. More specifically, the bias voltage at the first position is set not such that the electrostatic force created by the bias voltage is in equilibrium with the elastic force of the supporting member but such that the electrostatic force created by the bias voltage becomes smaller than the elastic force of the supporting member thereby assuring a stable holding force at the first position. As described above with reference to equation (2), the electrostatic force varies in inverse proportion to the square of the distance. Therefore, if the supporting member is set to have a proper displacement at the first position such that an elastic force is provided by the supporting member at the first position, then it is possible to drive the switching part in a stable fashion using a driving voltage smaller than a value which produces absolutely no equilibrium with the elastic force of the supporting member at any point. In this case, the driving voltage is allowed to produce equilibrium with the elastic force of the supporting member at one or more points, as long as there is no equilibrium point in the range between the first position and the second position. This technique allows the driving voltage to be reduced without applying a bias voltage. If this technique is combined with the application of bias voltage, then it is possible to further reduce the driving voltage.
Alternatively, the supporting member may be formed to be capable of supporting the switching part at a substantially central position between the first and second positions when no electrostatic force is exerted on the switching part, and the electrostatic driving means may include a first pair of electrodes for holding the switching part at the first position and a second pair of electrodes for holding the switching part at the second position. If a driving voltage is alternately applied to the first and second pairs of electrodes, a holding force can be provided by the electrostatic force at the respective positions. In this technique, each pair of electrodes is responsible for moving the switching part one-half the distance between the first and second positions. Because the distance between each pair of electrodes for creating the electrostatic force becomes one-half the distance in previous examples, the driving voltage can be greatly reduced, as can be understood from equation (2). This allows the driving voltage to be reduced even when no bias voltage is applied. Of course, this technique can be combined with the application of bias voltage to further reduce the driving voltage.
As described above, the optical switching device and the method of controlling the same according to the present invention allow the driving voltage to be reduced without changing either the distance between the first and second positions associated with the switching part or the elastic modulus of the supporting member. This makes it possible to drive the high-speed and high-contrast optical switching device using a small driving voltage. The control method according to the present invention is not limited to the optical switching device described above, but it can be applied to any spatial optical modulation device in which a switching element is mechanically moved thereby modulating light. Thus, it is possible to provide a spatial optical modulation apparatus capable of operating at a high speed with small power consumption at low cost.
Controlling the Attitude of the Switching Part
In the art of the optical switching device using evanescent light, it is always important to improve the operating speed. In view of the above, the present invention provides a technique of further increasing the operating speed of the optical switching device or the spatial optical modulation device which modules light by moving the switching part including the flat extraction plane.
The inventors of the present invention have investigated the operation of optical switching devices including an element in the form of a flat plate such as an extraction plane. The investigation has revealed that when the switching part moves a small distance at a high speed in the switching operation, the resistance of air or fluid such as an inert gas between the extraction plane of the switching part and the total reflection plane or the resistance of air or fluid which occurs when the switching part moves acts as a non-negligible force against the motion of the switching part, and that the operating speed can be greatly improved by reducing such resistance of the fluid. One technique of reducing the fluid resistance is to operate the optical switching device in a vacuum as described above with reference to one embodiment of the invention. However, to obtain a vacuum ambient in which the switching part or the driving part is disposed, an additional member such as a pressure-resistant container is required, which causes increases in the size of the device and cost. Furthermore, in production, an additional processing step is required to obtain the vacuum ambient. In the case of an optical switching device which can be used only in a vacuum ambient, if leakage occurs during operation, abrupt degradation in performance occurs or the optical switching device becomes inoperative. Thus, such a type of optical switching device has a problem associated with reliability. Thus, the present invention provides a technique of reducing the fluid resistance by controlling the motion (attitude) of the switching part, as described below. More specifically, in an early, intermediate, or final stage of the moving process, the driving means tilts the orientation of the extraction plane of the switching part with respect to a first direction in which the extraction plane faces when the switching part is at the first position. By tilting the switching part in the early stage of the moving process, it becomes possible for a fluid to smoothly flow into a space which is created as a result of separation of the switching part which occurs when the switching part start to move. That is, the fluid resistance can be reduced.
If the switching part is then moved while maintaining the tilted state, the inclination of the flat plane element with respect to the moving direction results in a reduction in the fluid resistance exerting on the moving switching part. The tilt of the switching part in the final stage of the moving process allows the fluid to smoothly escape from the space decreasing in volume as the switching part comes into the final resting position. Thus, the fluid resistance is reduced also in this case. As described above, the fluid resistance can be reduced by tilting the flat plane element at least in one of the early stage, main stage, and the final stage of the moving process of the switching part. If the flat plane element is tilted in all of the early stage, main stage, and the final stage of the moving process or in any two of those, a further reduction in the fluid resistance can be achieved. The reduction in the resistance during the movement of the switching part results in an increase in the moving speed and an increase in the modulation speed. Because this technique allows the fluid resistance to be reduced without reducing the pressure in the ambient in which the switching part is disposed or without needing a vacuum ambient, the optical switching device can be operated at a high speed in a common environment without having to placing the optical switching device in a pressure container. Thus, it is possible to provide a high-speed and high-reliability optical switching device at low cost.
In the optical switching device using evanescent light according to the present invention, when the extraction plane of the switching part in contact at the first position with the total reflection plane moves away from the total reflection plane or when the extraction plane of the switching part comes into contact with the total reflection plane, motion of the fluid is limited. This causes an increase in resistance against the movement of the switching part. This problem can be avoided by tilting the switching part in the early stage or the final stage of the movement process thereby making it possible for the fluid to flow into or escape from a space between the extraction plane and the total reflection plane. This allows a great improvement in the operating speed.
Of course, this method of controlling the attitude may also be applied to various types of spatial modulation devices other than the optical switching device using a flat plane element according to the invention.
The attitude of the switching part may be controlled by applying a driving force having a distribution asymmetric about the center of gravitation of the switching part to the switching part thereby bringing the switching part into an asymmetric state, that is, into a tilted state. Using this technique, it is possible to tilt the switching part in the early stage, the main stage, or the final stage of the moving process. One method of applying a driving force having an asymmetric distribution is to form the switching part such that it has a center of gravitation at a point shifted from the geometric center thereby allowing a driving force symmetric with respect to the geometric shape of the switching part to behave as a force asymmetric about the center of gravitation of the switching part thus tilting the switching part.
Another method is to form the driving means including a supporting member for elastically supporting the switching part such that the supporting member has an elastic modulus distribution which is, at least partially, asymmetric about the center of gravitation of the switching part thereby allowing a driving fore asymmetric about the center of gravitation of the switching part to be applied to the switching part. In the case where the driving means includes electrostatic driving means consisting of a first electrode disposed on the switching part and a second electrode disposed at a location opposing the first electrode, the shape of the first electrode or the second electrode or the distance between the first and second electrodes may be, at least partially, asymmetric about the center of gravitation of the switching part thereby allowing an asymmetric driving fore to be applied to the switching part.
Still another method is to divide the first or second electrode into a first and second parts which are asymmetric to each other about the center of gravitation of the switching part and apply electric power to the first and second parts in such a manner that the timing of applying electric power or the voltage of the electric power is different between the first and second parts thereby applying an asymmetric driving force to the switching part.
When the switching part is at the second position, it is not necessarily required that the orientation of the switching part is parallel to the orientation at the first position. If the switching part is adapted to rest at the second position while maintaining the tilted orientation, it is possible to make a smooth transition to or from a tilted state in the early stage, the main stage, and the final stage of the moving process. As a result, the fluid resistance is further reduced, and the operating speed is improved.
One method of maintaining the tilted orientation of the switching part at the second position is to form the supporting member of the driving means such that its elastic modulus becomes asymmetric about the center of gravitation of the switching part. Another method is to form the first and second electrodes such that the distance between the first and second electrodes becomes asymmetric. Still another method is to form the supporting post the switching part comes into contact with at the second position such that the distance between the supporting post and the switching part becomes asymmetric about the center of gravitation of the switching part.